The present invention relates to a panoramic tomography X-ray apparatus, especially for dental photography. This apparatus comprises a fixed frame, to which another rotary frame having an X-ray tube at one end and a film cartridge device for X-ray film at the other end, is suspended, e.g. through bearings. An object to be photographed, e.g. a patient, can be positioned between the X-ray tube and the film cartridge. The apparatus has a drive mechanism, by means of which the suspended rotary frame is rotated in a plane, preferably a horizontal plane, for taking panoramic X-ray photographs. The present invention is also directed to a method for taking panoramic photographs, such as X-rays of teeth.
The design and operating principle of most panoramic X-ray apparatus involve turning the X-ray path about the head of a patient, in such a manner that the dental arch will be photographed as a flat picture on a moving film.
In order to make the object being photographed sharp and the structures in front and behind the object invisible by "fogging" out of focus, the lateral velocity of the film with regard to the ray bundle must be equal to the sweep velocity of the ray bundle in the object multiplied by the magnification ratio. The magnification is determined by the ratio of the distance between the focus and the film, to the distance between the focus and the object.
The thickness of the layer being photographed sharply, is directly proportional to the distance of the instantaneous center of rotation from the film plane, and inversely proportional to the magnification and to the width of the ray bundle. A singular consideration, from the point of view of how the object will be represented, is how the focus, the object, and the film level are located with respect to one another. The instantaneous center of rotation is significant only through the sweep velocity.
It is possible to form the basic equation of panoramic photography, based on what has been outlined above:
V.sub.1 /V.sub.0 =L.sub.1 /L.sub.0, and PA0 V.sub.0 =.omega.r, where PA0 L.sub.0 =the distance from the focus F to the point being photographed a given moment, PA0 L.sub.1 =the distance from the focus F to the film level, PA0 .omega.=the angular velocity of the rotary movement about the center of rotation, PA0 r=the distance of the point being photographed from the instantaneous center of rotation, and PA0 v.sub.1 =the velocity of an image point on the picture level (film level). PA0 orthogonality, aimed at preventing adjacent teeth at any point of the dental arch from being photographed on top of one another. Therefore, the ray bundle must sweep the object as perpendicularly as possible, i.e. orthogonally against the dental arch; PA0 constant magnification, i.e. the magnification must be the same all over the dental arch, with the distance from the layer being photographed to the film level being maintained constant throughout the entire rotation; PA0 evenness of movement, which has been difficult to maintain since it has been necessary to move the instantaneous center of rotation during the exposure. The projection must be such that the instantaneous center of rotation may move without discontinuities which could create excessive acceleration, thus harming the quality of the picture; and PA0 minimization of the radiation load to the patient, for which the projection must be such that excessive radiation will not be applied to a single point on the patient. PA0 so called revolving slides move in grooves, which makes it very difficult, even impossible, to operate without play; PA0 the stability of the apparatus in the horizontal plane is questionable at such points of the rotary movement in which the revolving slide guide grooves are at an unfavorable angle towards one another; PA0 a method of transmitting rotary movement leads to improper rotation of the drive wheel (i.e. axial displacement); PA0 in the particular arrangement described in this patent, in which one of the revolving slide grooves is replaced by a crank, the control of the revolving cranks in the groove is critical from the point of view of the result of the exposure, since the revolving slide does not stably follow either one of the groove edges; and PA0 in the last-mentioned particular arrangement of the above reference, transmission of the rotary movement can be arranged so that the drive will be proper and the drive wheel runs along a circular track, however the design will be complex, bulky, and expensive.
The most important objects of photography by a panoramic tomography X-ray apparatus, are the dental arch and the temporomandibular joints (there are other important objects of photography too). A fixed center of rotation must be abandoned in order to generate as useful a projection as possible of the object, and the least questionable mechanical arrangement. Moving the position of the center of rotation depends, among other factors, on the following:
It should be possible to manufacture an X-ray source of the apparatus and the rotary mechanism for the film cartridge to create the projection meeting the requirements specified above, within reasonable cost so that there will be no play or other inaccuracies detrimental to the photography. In order to create the desired projection, the mechanism must therefore move the center of rotation in a horizontal plane. The mechanism must also vertically support the entire equipment. Therefore, the vertical bearing system must be stable and without play in order to provide a good photograph.
In the prior art, orthogonality has been satisfactorily maintained in the dental arch area. However, almost no attention has been paid to the temporomandibular joint. With commercially-available apparatus, it is not possible to maintain the magnification ratio constant, a shortcoming which can be seen in the tomography exposures in a manner such that the front part of the dental arch and/or the entire dental arch are photographed with noticeably higher magnification ratios, than the temporomandibular joints. The reason for this, is that in the known apparatus, the film cartridge comes much closer to the object being photographed in the side region, than in the frontal area.
Additionally, the rotary movement in the prior art has been such that, during the entire exposure, the center of rotation is located in the area outlined by the temporomandibular joints or near this area, resulting in a high local radiation load in this particular area. For the same reason, the temporomandibular joints are photographed at an oblique angle, which is an obvious drawback for diagnostic purposes.
The most relevant prior art will now be described in connection with the following references:
Finnish patent application No. 833,754 illustrates panoramic tomography equipment, in which the constant magnification problem is solved in one manner. This reference, however, takes no position on how to produce the projection. A solution in accordance with British Pat. No. 1,594,499, referred to in the above Finnish application, produces a projection in which the temporomandibular joints are photographed at a highly oblique angle (not orthogonally). In an apparatus in accordance with the above application, the movement parallel with the ray bundle, required by constant magnification, is created by a linear bearing system which is structurally a difficult and expensive solution.
U.S. Pat. No. 4,264,820 deals with an X-ray method and apparatus, in which attention has been paid to the constant magnification problem. However, even this patent has no view on the projection to the temporomandibular joint area. Orthogonality in this reference is only examined in the dental arch area. Two systems are described for attaining constant magnification. However, the following drawbacks can be perceived in both these systems: